The present invention relates to connection control of virtual channels in an ATM (Asynchronous Transfer Mode) communication network, and particularly to a method of connection reservation control of an ATM communication network and an ATM exchange for performing the connection reservation control, wherein virtual channels are left reserved without executing their release.
In an ATM communication network, it is better for the resource efficiency to release a virtual connection every time when a communication, which has been performed thereby, is finished. However, somewhat complicated procedures are required for establishing and releasing the virtual connection.
FIG. 10 is a sequence chart illustrating procedures to be performed for establishing and releasing a virtual connection between a transmitter terminal TE1 and a receiver terminal TE2 through a transmitter-side exchange 500, an intermediate exchange 501 and a receiver-side exchange 502.
For establishing a virtual connection, the transmitter terminal TE1 transmits a call setup request SETUP to the transmitter-side exchange 500. The transmitter-side exchange 500 outputs an address message IAM (Initial Address Message) to the intermediate exchange 501, which also outputs the same address message IAM to the receiver-side exchange 502. The receiver-side exchange 502 transmits a call setup request SETUP to the receiver terminal TE2. By performing such a sequence of procedures as above described, a virtual connection is established.
In a similar way, a disconnection notification DISC or releasing request REL should be exchanged between the transmitter terminal TE1, the transmitter-side exchange 500, the intermediate exchange 501, the receiver-side exchange 502 and the receiver terminal TE2, for releasing the virtual connection.
FIG. 11 is a block diagram illustrating a configuration example of an ATM exchange taking charge of performing above procedures for establishing or releasing the virtual connection, which is described in a Japanese patent application laid open as a Provisional Publication No. 160851/""93.
Referring to FIG. 11, a first signal processing unit 94 performs necessary procedures for connecting a virtual channel between a terminal 100 and the ATM exchange 20. A second signal processing unit 95 performs necessary procedures for connecting or disconnecting a virtual channel between the ATM exchange 20 and a neighboring node of an ATM network 400.
Receiving a call setup request from the terminal 100 through a terminal interface 81 and the first signal processing unit 94, a call processing unit 96 selects a virtual channel of a quality of service demanded by the terminal 100 and transmits an initial address message towards the ATM network 400 through the second signal processing unit 95.
According to the virtual channel selected by the call processing unit 96, a channel switch control unit 92 controls a channel switch 85 to switch ATM cells from the terminal 100 according to the selected channel.
In the example of FIG. 11, a cross connect processing unit 97 takes charge of selecting an appropriate detour path for each group of virtual channels when a virtual path assigned to the group is congested. According to the selection of the cross connect processing unit 97, a path switch control unit 93 controls a path switch 86 to switch the virtual channel towards an appropriate port, by way of which ATM cells from the terminal 100 are transmitted through a network interface 82.
Thus, a virtual channel is connected to the terminal 100 in the ATM exchange 20.
When a releasing request of the virtual connection is received by the call processing unit 96 from the terminal 100, the call processing unit 96 retrieves the concerning virtual channel and transfers the releasing request towards the ATM network 400 through the second signal processing unit 95. The channel switch control unit 92 releases connection of the virtual channel in the ATM exchange 20 by deleting data of the virtual channel from the switching table of the channel switch 85.
Such procedures as performed in the ATM exchange 20 should be executed in each of: the transmitter-side exchange 500, intermediate exchange 501 and the receiver-side exchange 502 of FIG. 10, for establishing a virtual connection between the transmitter terminal TE1 and the receiver terminal TE2. In other words, the establishment or release of a virtual connection is performed through connecting or disconnecting two neighboring ATM exchanges or an ATM exchange with a terminal with a virtual channel, one by one, by exchanging connection requests or releasing requests between them.
Conventionally, the virtual connection is established or released as above described every time a call setup or a call disconnection is requested from a terminal. However, there have been following problems in such connection control.
First, even for a short suspension of a communication, the virtual connection established for the communication should be released, by disconnecting the concerning virtual channels in every ATM exchange which participates in the virtual connection through exchanging releasing requests between neighboring nodes. And, secondly, even when the virtual connection, which is once released, is to be established again between the same pair of terminals, the same procedures as performed for firstly establishing the virtual connection should be repeated for connecting a virtual channel in every ATM exchange by exchanging connection requests among them.
Therefore, when the call setup request or the call disconnection request occurs frequently, the procedures for connecting or disconnecting virtual channels in the ATM exchanges increase, resulting in degradation of connection establishment ability of the ATM communication network. When the call setup requests and the call disconnection requests of the same pairs of terminals are repeated frequently, the concerning ATM exchanges are forced to repeat duplicated procedures.
A method of resolving this problem is to establish PVCs (Permanent Virtual Connections) between pairs of terminals. However, the PVC is not flexible and therefore uneconomical to be assigned for connecting terminals whereof communication may be interrupted for a long time.
As a prior art for dealing with this problem, a terminal adapter is disclosed in a Japanese patent application laid open as a Provisional Publication No. 239950/""92. In this prior art, when the terminal adapter, which is connecting a terminal A to a communication network for communicating with a terminal B through the network, for example, receives a connection release request of the communication channel from the terminal A, the terminal adapter returns a connection release acknowledge signal to the terminal A. However, the terminal adapter reserves the communication channel being connected for a fixed period without releasing the connection.
Therefore, when a call setup with the terminal B is requested again by the terminal A during the fixed period, the communication between the terminal A and the terminal B can be reopened at once without any procedure for re-establishing the communication channel in the network.
In the prior art, the terminal adapter has two ports and can connect the terminal A also to another terminal C through the network during the fixed period where the communication channel with the terminal B is reserved being connected.
This prior art is somewhat effective for reducing load of exchanges in the communication network for establishing and releasing the virtual connection, in the case where the terminal A repeats communication with the terminal B directly, or alternately with the terminal B and the terminal C, for example, within an interval shorter than the fixed period.
However, in the prior art, the connection reservation control is performed by the terminal adapter. Therefore, there have been following problems.
The first problem is that a number of communication channels to be reserved is limited. When the terminal A intends to communicate with a terminal B, a terminal C and a terminal D in turn, for example, only one channel can be reserved without releasing, in case the number of the available communication channels is limited in two.
The second problem is that the connection reservation control should be performed without taking in consideration the usage status of the connection resources. Therefore, the connection reservation should be cancelled uniformly after the fixed period, even when there are left a lot of connection resources unused, or the connection reservation should be retained uniformly until the fixed period passes, even when all the connection resources are used.
Therefore, a primary object of the present invention is to provide a method of connection reservation control of an ATM network wherein any number of virtual connections can be reserved for a terminal without releasing the connections, as long as sufficient connection resources are left unused.
In order to achieve the object, a method of connection reservation control of an ATM communication network according to the invention comprises steps of:
reserving a virtual connection which has been used for a communication between a transmitter terminal accommodated in a first ATM exchange and a receiver terminal accommodated in a second ATM exchange of the ATM communication network without releasing the virtual connection when a connection release request of the virtual connection is requested by the transmitter terminal; and
reopening the communication between the transmitter terminal and the receiver terminal by way of the virtual connection which has been reserved when a connection setup request of the same connection with the virtual connection which has been reserved is requested by the transmitter terminal.
The step of reserving comprises steps of:
registering information of the virtual connection to be reserved in a reserved-connection table of the first ATM exchange; and
transmitting an OAM (Operation, Administration and Maintenance) cell of communication suspension from the first ATM exchange to the second ATM exchange by way of the virtual connection to be reserved;
The step of reserving further comprises steps of:
registering information of the virtual connection to be reserved in a reserved-connection table of the second ATM exchange by receiving the OAM cell of communication suspension; and
transmitting an ATM cell of a connection release request of the virtual connection from the second ATM exchange to the receiver terminal.
The step of reopening comprises steps of:
deleting the information of the virtual connection which has been reserved from the reserved-connection table of the first ATM exchange; and
transmitting an OAM cell of communication reopen from the first ATM exchange to the second ATM exchange by way of the virtual connection which has been reserved.
The step of reopening further comprises steps of:
deleting the information of the virtual connection which has been reserved from the reserved-connection table of the second ATM exchange by receiving the OAM cell of communication reopen; and
transmitting an ATM cell of a connection setup request of the virtual connection from the second ATM exchange to the receiver terminal.
Therefore, any number of virtual connections can be reserved for a terminal without releasing the connections, as long as sufficient connection resources are left unused, and the reserved connection can be released at any time according to need, in the ATM exchange according to the invention.